Tham khảo tài liệu 'metal_foams episode 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Making metal foams 9 solidify. The thermal gradient in the foam determines how long the foam remains liquid or semi-solid and thus the extent of drainage. Low relative density closed-cell foams can be produced by carefully controling the gasinjection process and the cooling rate of the foam. Various techniques can be used to draw-off the foam and create large up to 1 m wide and m thick slabs containing closed cell pores with diameters between 5 and 20 mm. NORSK-HYDRO and CYMAT the latter using a process developed by ALCAN in Canada supply foamed aluminum alloys made this way. This approach is the least costly to implement and results in a foam with relative densities in the range to . It is at present limited to the manufacture of aluminum foams. Gas-releasing particle decomposition in the melt Metal alloys can be foamed by mixing into them a foaming agent that releases gas when heated. The widely used foaming agent titanium hydride TiH2 begins to decompose into Ti and gaseous H2 when heated above about 465 C. By adding titanium hydride particles to an aluminum melt large volumes of hydrogen gas are rapidly produced creating bubbles that can lead to a closedcell foam provided foam drainage is sufficiently slow which requires a high melt viscosity. The Shinko Wire Company has developed an aluminum foam trade named Alporas using this approach Figure . The process begins by melting aluminum and stabilizing the melt temperature between 670 and 690 C. Its viscosity is then raised by adding 1-2 of calcium which rapidly oxidizes and forms finely dispersed CaO and CaAl2O4 particles. The melt is then aggressively stirred and 1 - 2 of TiH2 is added in the form of 5-20 inn diameter particles. As soon as these are dispersed in the melt the stirring system is withdrawn and a foam is allowed to form above the melt. Control of the process is achieved by adjusting the overpressure temperature and time. It takes typically about ten minutes to totally decompose .
